Claude.ai, set to Opus xhigh, attach file, "Try to decode the file format of this save game from Name of Game. Do research if necessary"

If the file's too big, you'll probably need to download a harness/local app. If you go local route, make sure you have a backup in case the LLM breaks the file

The local route it can also try to poke at the binaries that read/write the file so it would have a bit more to work with going that route